Finite-frequency sensitivity kernels for two-station surface wave measurements

We present a method for the computation of finite-frequency sensitivity kernels for two-station surface wave measurements. It is based on the combination of spectral-element modelling of seismic wave propagation, adjoint techniques and traveltime estimates from two-station cross-correlations of seismograms. The analysis of sensitivity kernels for a 1-D earth model resulted in two major conclusions: (1) The finite-frequency sensitivity is zero along the interstation ray path for group velocity measurements obtained by cross-correlations. It follows that interstation group velocity measurements should not be made by cross-correlation if a ray-based interpretation is used. (2) Although sensitivity along the interstation ray path is dominant for phase velocity measurements, sensitivity far from the ray path can be large, depending on the details of the source–receiver geometry. The complexities of finite-frequency two-station sensitivity kernels should be taken into account to avoid misinterpretations and to improve the quality of tomographic inversions.